Hydro Tail. 

Prices are in Australian Dollars.
  
First at Kona.  If you need to mount bottle cages, spare tires, gas cylinders or a tow bar off the rear of your saddle, you need the Hydro Tail.

This is a very clever piece of carbon fibre and its also very light.  Our crash test pilot went the distance to ensure that the Hydro Tail is the best hydration carrying system - period.